1) Field of the Invention
The present invention relates to an optical pickup device and an optical recording medium reproducing device.
2) Description of the Related Art
Conventionally, there is known an optical pickup device that reads information recorded in an optical recording medium such as a compact disc (CD) or a digital video disc (DVD) by a laser beam. The optical pickup device at least includes a light source that outputs a laser beam, an optical system that focuses laser beams output from the light source on a recording layer in the optical recording medium, and a light receiving unit that receives a return light from the recording layer. The optical pickup device uses the mechanism to irradiate a laser beam on the optical recording medium and to read information recorded in the recording layer based on an intensity of the return light acquired in the light receiving unit, or the like.
In the optical pickup device, a laser beam output from the light source is required to be focused on a spot having a predetermined diameter in the recording layer to avoid occurrence of reading error or the like as much as possible. Particularly, in recent years, a recording density is remarkably improved along with a larger capacity of the optical recording medium, thus a smaller spot diameter is required to be realized. Therefore, there has been proposed an optical pickup device that mounts a phase correcting unit for avoiding occurrence of a wavefront aberration which causes enlargement of a spot diameter as well as a design feature of the optical system.
The phase correcting unit is positioned between the light source and the optical recording medium, and is formed to include a liquid crystal panel, for example. A liquid crystal material contained in the liquid crystal panel has a structure where an index of refraction changes according to an applied voltage so that an appropriate applied voltage is selected to compensate for a phase difference in a laser beam that causes a wavefront aberration and to restrict occurrence of the wavefront aberration to a permissible degree (see Japanese Patent Application Laid-Open No. H9-128785).
However, since there are many factors that may cause the wavefront aberration, in the optical pickup device having a mechanism that uses a plurality of laser sources to read information recorded in a plurality of kinds of recording mediums, it is difficult to compensate for a phase difference in a laser beam and to restrict occurrence of the wavefront aberration only by applying an appropriate applied voltage to a liquid crystal panel. Particularly, in using Blue ray Disk (BD) and DVD, the wavefront aberration easily influences reading of these optical recording mediums.